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Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details) $ in Thousands		Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
AFS Fixed Income Securities		$ 6,598,822	$ 6,709,976
Equity Securities		344,583	335,537
Short-term investments		256,712	447,863
Net Asset Value	[1]	96,400	83,600
Fair Value, Recurring [Member]
AFS Fixed Income Securities		6,598,822	6,709,976
Equity Securities		344,583	335,537
Short-term investments		256,712	447,863
Total assets measured at fair value		7,200,117	7,493,376
U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		127,093	130,458
Foreign Government [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		16,703	15,860
Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		$ 1,125,431	$ 1,189,308
Corporate Securities [Member] | Weighted Average | Measurement Input, Credit Spread | Valuation Technique, Discounted Cash Flow
Debt Securities, Available-for-sale, Measurement Input		(0.011)	(0.012)
Corporate Securities [Member] | Maximum | Measurement Input, Credit Spread | Valuation Technique, Discounted Cash Flow
Debt Securities, Available-for-sale, Measurement Input		0.053	0.030
Corporate Securities [Member] | Minimum | Measurement Input, Credit Spread | Valuation Technique, Discounted Cash Flow
Debt Securities, Available-for-sale, Measurement Input		(0.044)	0.003
Corporate Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		$ 2,387,740	$ 2,573,603
Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Weighted Average | Measurement Input, Credit Spread | Valuation Technique, Discounted Cash Flow
Debt Securities, Available-for-sale, Measurement Input		(0.019)	(0.021)
Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Maximum | Measurement Input, Credit Spread | Valuation Technique, Discounted Cash Flow
Debt Securities, Available-for-sale, Measurement Input		0.080	0.080
Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Minimum | Measurement Input, Credit Spread | Valuation Technique, Discounted Cash Flow
Debt Securities, Available-for-sale, Measurement Input		0.0001	0.007
Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		$ 1,417,214	$ 1,350,814
Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		877,959	776,252
Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		646,682	673,681
Common Stock [Member] | Fair Value, Recurring [Member]
Equity Securities	[1]	342,572	333,449
Preferred Stock [Member] | Fair Value, Recurring [Member]
Equity Securities		2,011	2,088
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		31,671	60,615
Equity Securities		248,189	251,934
Short-term investments		250,938	442,723
Total assets measured at fair value		530,798	755,272
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		31,671	60,615
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Common Stock [Member] | Fair Value, Recurring [Member]
Equity Securities	[1]	246,178	249,846
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member]
Equity Securities		2,011	2,088
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		6,305,747	6,398,079
Equity Securities		0	0
Short-term investments		5,774	5,140
Total assets measured at fair value		6,311,521	6,403,219
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		95,422	69,843
Significant Other Observable Inputs (Level 2) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		16,703	15,860
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		1,118,186	1,181,563
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		2,264,938	2,459,476
Significant Other Observable Inputs (Level 2) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		1,286,296	1,225,905
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		877,959	776,007
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		646,243	669,425
Significant Other Observable Inputs (Level 2) [Member] | Common Stock [Member] | Fair Value, Recurring [Member]
Equity Securities	[1]	0	0
Significant Other Observable Inputs (Level 2) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member]
Equity Securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		261,404	251,282
Equity Securities		0	0
Short-term investments		0	0
Total assets measured at fair value		261,404	251,282
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government and Government Agencies [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Foreign Government [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		7,245	7,745
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		122,802	114,127
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		130,918	124,909
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		0	245
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Recurring [Member]
AFS Fixed Income Securities		439	4,256
Significant Unobservable Inputs (Level 3) [Member] | Common Stock [Member] | Fair Value, Recurring [Member]
Equity Securities	[1]	0	0
Significant Unobservable Inputs (Level 3) [Member] | Preferred Stock [Member] | Fair Value, Recurring [Member]
Equity Securities		$ 0	$ 0
[1] Investments amounting to $96.4 million at March 31, 2022, and $83.6 million at December 31, 2021, were measured at fair value using the net asset value per share (or its practical expedient) and have not been classified in the fair value hierarchy. These investments are not redeemable and the timing of liquidations of the underlying assets is unknown at each reporting period. The fair value amounts in this table are intended to permit reconciliation of the fair value hierarchy to total assets measured at fair value.

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